1. Field of the Invention
The present invention relates to a cable entrance module, and, in particular, to a cable entrance module which is applied to an optical subscriber-line network unit installed aerially.
FIG.1 roughly shows a communication system to which an optical subscriber-line network unit is applied. This communication system is proposed as a near future system for coping with multimedia use. A feature of this system is that an optical cable extends to a position near to houses.
Telephone stations 10 and 11 are connected by an optical cable 12. From the telephone station 11, a multi-conductor trunk-line optical cable 13 extends via branch units 30 while being installed on poles 14. An aerially installed optical subscriber-line network unit 20 is set aerially outdoors for every ten houses, for example. In the branch unit 30, a one-conductor drop cable 31 branches off from the trunk-line optical cable 13, and the drop cable 31 enters the optical subscriber-line network unit 20. The optical subscriber-line network unit 20 has an arrangement in which, in a box 32, a light-electricity converter 21, a multiplexer/demultiplexer 22, and ten and several subscriber channel units 23 are contained. The extending end of the drop cable 31 is connected to the optical subscriber-line network unit 20, and, also, a power line 17 is connected to the optical subscriber-line network unit 20. Power is supplied to the optical subscriber-line network unit 20 through the power line 17 so that the optical subscriber-line network unit 20 may operate. One outside-line metal cable 18 extends from each of the subscriber channel units 23, and, thus, ten and several outside-line metal cables 18 extend from the optical subscriber-line network unit 20. Each outside-line metal cable 18 extends to a respective one of houses 15, and is connected to a communication terminal such as a telephone, a facsimile machine or the like.
In this communication system, because the drop cable 31 extends to the position near to the houses 15 and the length of each of the outside-line metal cables 18 is short, it is possible to transmit a large amount of information in comparison to the conventional case. Further, in addition to voice transmission, image transmission is possible. Furthermore, a digital signal and so forth are transmitted.
In the above-described optical subscriber-line network unit 20, a drop-cable entrance module 33 is attached to a side plate 32a of the box 32. The drop cable 31 is inserted through the inside of the drop-cable entrance module 33, and, then, enters the box 32.
As shown in FIG. 2A, the drop cable 31 includes an optical fiber 40 at the center thereof, and piano wires 41 and 42 on both sides thereof. The entirety of the drop cable is covered by a covering portion 43 made of vinyl chloride. Further, notch grooves 44 and 45 are formed in the drop cable 31 on the top and bottom sides facing the optical fiber 40. The drop cable 31 has an approximately figure-eight-shaped cross section, as shown in the figure. The drop cable 31 enters the box 32 through the drop-cable entrance module 33, and, then, as shown in FIG. 1, is split into two by using the notch grooves 44 and 45. Then, the optical fiber 40 is exposed, and a terminal treatment, in which the covering portion 43 is cut and removed, and the piano wires 41 and 42 are exposed, is performed. Then, the ends of the piano wires 41 and 42 are fixed to the box 32 by using fixing screws 50 and 51. The optical fiber 40 is connected to the light-electricity converter 21 by using an optical connector.
The piano wires 41 and 42 are provided for the purpose of receiving a pulling force when the drop cable 31 is pulled and preventing the pulling force from being applied to the optical fiber 40. The notch grooves 44 and 45 are formed for the purpose of enabling easy splitting of the drop cable 31 into two as mentioned above. The reason why the notch grooves 44 and 45 are formed on the top and bottom sides facing the optical fiber 40 is that the optical fiber 40 is exposed when the drop cable 31 is split into two at the end thereof as mentioned above.
Because the optical subscriber-line network unit 20 is set outdoors, it is demanded, in order for the optical subscriber-line network unit 20 to have a high reliability, that the drop-cable entrance module 33 is sufficiently waterproof so that rain water drops flowing on the drop cable 31 do not enter the optical subscriber-line network unit 20 in a condition in which the drop cable 31 has been inserted through the drop-cable entrance module 33.
The waterproof structure of the drop-cable entrance module 33 should be such that the cross section of the drop cable 31 is not a circle and has a special shape as mentioned above.
2. Description of the Related Art
FIG. 3A shows a drop-cable entrance module 60 in the related art. The drop-cable entrance module 60 includes a sleeve 62 made of rubber having a tunnel 61, and a pressing mechanism (not shown in the figure) which presses the outer circumferential surface of the sleeve 62 as a result of screws being tightened. The cross section of the tunnel 61 has a figure-eight shape corresponding to the cross section of the drop cable 31. Projections 63 and 64 project inwards from the top and bottom of the tunnel 61, respectively. The projections 63 and 64 extend along the tunnel 61. The drop-cable entrance module 60 is attached to the side plate 32a of the box 32 of the optical subscriber-line network unit 20.
The drop cable 31 is inserted through the tunnel 61 of the sleeve 62 as shown in FIG. 3A. Then, the sleeve 62 is pressed by the pressing mechanism. As a result, as shown in FIG. 3B, the inner wall of the tunnel 61 makes contiguous contact with the outer circumferential surface of the drop cable 31, so that no gap is formed between the inner wall of the tunnel 61 and the outer circumferential surface of the drop cable 31. Thus, a waterproof structure is formed. Thereby, rain water drops flowing on the drop cable 31 are prevented from entering the optical subscriber-line network unit 20.
However, there may be a case where, as shown in FIG. 3C, the drop cable 31 is located at a position shifted rightward a little from the center of the tunnel 61. When the pressing mechanism presses the sleeve 62 in this condition, as shown in FIG. 3D, spaces 65 may remain between the outer surface of the drop cable 31 and the inner wall of the tunnel 61. The reason why the spaces 65 remain is that the cross section of the drop cable 31 has the special shape having the notch grooves 44 and 45, and the notch grooves 44 and 45 are not aligned with the projections 63 and 64, respectively. In a case where the drop cable 31 is located in a position shifted leftward a little from the center of the tunnel 61, similar spaces may remain. When spaces remain, a completely waterproof structure cannot be provided.
Thus, the drop-cable entrance module 60 in the related art is problematic in view of reliability.